Benchmark optimal settings documentation
Use the blenchmark Addon: http://blenchmark.com/article/benchmark-your-cpu-or-gpu. The istructions on how to install and use it are explained very clearly on the site

There are two basic settings: one for CPU and one for GPU, don't use the one for CPU to test a GPU.

The idea behind the blenchmark is to have a standardized set of parameters by which to compare systems. Just set the compute device, leave all other setting untouched and let it render.

Changing the settings for each machine would result in a comparison like watermelons and oranges.

Note that it is quite easy to render faster than the average results, as the blenchmark is designed to work on many different types of hardware and Is not optimized for any particular one and it is set to work on a very wide range of hardware.